1. Field of the Invention
This invention relates to television receiving apparatus and more particularly to a television receiving apparatus wherein a video buzzing sound is prevented from being produced by a video signal turning round into a power source system.
2. Description of the Related Art
As sound receiving systems of television receiving apparatus, there are a split carrier (or separate carrier) detecting system and intercarrier detecting system.
The intercarrier detecting system is a system wherein a sound intermediate frequency (for example, of 54.25 MHz) and a video intermediate frequency (for example, of 58.75 MHz) converted by a tuner are amplified by the same intermediate frequency amplifying circuit and are detected and a beat component (4.5 MHz) of the video intermediate frequency (58.75 MHz) and sound intermediate frequency (54.25 MHz) included together with a video signal in the output of the detecting circuit is utilized as a sound intercarrier signal.
FIG. 6 shows an essential part of a television receiving apparatus of a conventional intercarrier detecting system.
In FIG. 6, an RF television signal input into an antenna 1 is input into an electronic tuning tuner (mentioned as an ET tuner hereinafter) 2 in which a desired channel signal is selected from among the RF signals. An intermediate frequency signal (mentioned as an IF signal hereinafter) from the ET tuner 2 is amplified and video-detected by a video intermediate frequency circuit (mentioned as a video IF circuit hereinafter) 4. The video-detected video signal is fed to a sound intermediate frequency circuit (mentioned as a sound IF circuit hereinafter) 7 and video amplifying circuit 8. The video signal is amplified by the video amplifying circuit 8 and is output. At the same time, a sound intercarrier signal included in the video signal is amplified and sound FM-detected by the sound IF circuit 7. In the here shown apparatus, the power source for the local oscillating circuit of the ET tuner 2, the power source of the video IF circuit 4 and the power source of the video amplifying circuit 8 are obtained from the same power source circuit 9.
Now, in case the power source for the local oscillating circuit of the ET tuner 2, the power source of the video IF circuit 4 and the power source of the video amplifying circuit 8 are driven by the same power source circuit 9 as mentioned above, with the variation of the video signal current, a slight ripple voltage will be produced in the power source line, the local oscillation output of the ET tuner 2 will fluctuate because of this ripple voltage, the sound IF signal in the output of the ET tuner 2 will be also influenced by the video signal, as a result, the sound intercarrier signal obtained from the video IF circuit 4 will be influenced and, when this is sound-detected, a video buzzing sound will be generated in the television sound to disadvantage.
On the other hand, the split carrier detecting system wherein a sound intermediate frequency (for example, 54.25 MHz) and a video intermediate frequency (for example, 58.75 MHz) converted by a tuner are amplified respectively by exclusive amplifying circuits and are detected is different from the intercarrier detecting system wherein a sound signal is detected from a beat component (4.5 MHz) of a video intermediate frequency (58.78 MHz) and sound intermediate frequency (54.25 MHz). In the split carrier detecting system, fundamentally no buzzing sound is generated by the video signal. Therefore, this system is extensively used as a high quality sound detecting system.
In FIG. 7 is shown an essential part of a television receiving apparatus of a conventional split carrier detecting system.
In FIG. 7, an RF television signal input into an antenna 1 is input into an ET tuner 2 in which a desired channel signal is selected from among RF signals. An IF signal from the ET tuner 2 is input into a filter 3 and is here separated into a video IF signal and a sound IF signal. Then, the video IF signal is amplified and video-detected by a video IF circuit 4. The sound IF signal is mixed in a mixing circuit 6 with a local oscillating signal from a second local oscillating circuit 5 and is converted to a second sound IF signal (4.5 to 10.7 MHz) and is amplified and sound-detected with a sound alone in a sound IF circuit 7. The video signal video-detected in the video IF circuit 4 is amplified in the video amplifying circuit 8 and is output. In the apparatus shown here, the power source of the local oscillating circuit of the ET tuner 2, the power source of the video IF circuit 4 and the power source of the video amplifying circuit 8 are obtained from the same power source circuit 9.
The same as in FIG. 6, in FIG. 7, too, in case the power source of the local oscillating circuit of the ET tuner 2, the power source of the video IF circuit 4 and the power source of the video amplifying circuit 8 are driven by the same power source, a slight ripple voltage will be produced in the power source line with the variation of the video signal current, the local oscillating output of the ET tuner 2 will fluctuate because of this ripple voltage, the sound IF signal in the output of the ET tuner 2 will be modulated in the frequency by the video signal and, when it is sound-detected, a video buzzing sound will be generated in the television sound to disadvantage.
Thus, in the television receiving apparatus, the video buzzing sound is generated because the local oscillating output of the local oscillating circuit of the ET tuner 2 has a sensitivity fluctuating under the influence of the ripple voltage of a .mu.V order and the video signal turns round as a power source ripple into the ET tuner 2 side through the power source system and, as a result, influences the sound signal.
In order to reduce the above mentioned ripple voltage, briefly the capacity of a decoupling condenser for removing ripples usually connected to a power source line may be made large but, in the general circuit apparatus in which power source lines are connected to various load circuits, the load impedance is so low that the effect of removing ripples for the increase of the condenser capacity is small and the desired characteristics are hard to obtain.
Therefore, the ripple removing filter is strengthened exclusively for the power source for the local oscillating circuit of the ET tuner or another power source circuit 10 is provided besides the power source circuit 9 as connected to the main power source as shown in FIG. 8 and is made a power source for the local oscillating circuit of the ET tuner 2. The power source of the ET tuner 2 is separated from the power sources of the video IF circuit 4 and video amplifying circuit 8, the video signal is prevented from turning round into the ET tuner 2 side through the power source system and the video buzzing sound is prevented from being generated.
However, in the above mentioned method, the power source system is divided into two systems of power sources, an excess filter for removing power source ripples is required and therefore there have been problems that the space on the circuit substrate increases and the cost is high.